code: def on_step_begin(self, iteration, trials, **info):
      node_count = len([n for n in ray.nodes() if n["Alive"]])
      self.node_counts.append(node_count)
  
file_path: ray/release/cluster_tests/workloads/tune_scale_up_down.py
imports: from collections import Counter
  import time
  import ray
  from ray import train, tune
label_desc: Use image node_id 2 for calling the NodeCountCallback obj's underlying member method code with example usage: obj.on_step_begin(iteration, trials) without return types